BANGKOK — A Thai military court sentenced a businessman to 25 years in prison on Tuesday on charges of defaming the country's monarchy in what appears to be the longest sentence handed down in recent years for the crime of lese majeste, a civil liberties lawyer said.

Yingcheep Atchanont of the Thai Lawyers for Human Rights Center said the court in Bangkok found Thiensutham Suthijitseranee guilty on five counts of lese majeste for postings he made on Facebook, and handed him a 10-year sentence for each count. It cut the total 50-year term in half because Thiensutham pleaded guilty to the charges.

Thailand's lese majeste law is considered the harshest in the world, with those accused of defaming, insulting or threatening the monarchy facing jail terms ranging from three to 15 years on each count.

Lese majeste cases used to be rare, but have increased in the past decade as concerns have grown over the eventual succession process for King Bhumibol Adulyadej, now 87 and ailing. Political polarization enmeshing the monarchy has also led to more outspokenness on the subject.

The news website Prachatai described Thiensutham as a Bangkok businessman and a member of the "Red Shirt" political grouping, which comprises supporters of former Prime Minister Thaksin Shinawatra, who was ousted by a military coup in 2006 after being accused of corruption and disrespect for the king. It said Thiensutham was arrested last December.

Prachatai said Thiensutham's earliest cited offense was a Facebook posting in July 2014, two months after the military staged another coup against a government formed by Thaksin's sister, Yingluck Shinawatra. It said the Facebook postings, made under the name Yai Daengdueat, included comments critical of Thailand's new military regime.

"The defendant insulted the beloved and revered Thai monarchy," Prachatai quoted the judge saying. "The sentence handed down by the court is already light."

After last year's coup, the military decreed that any new cases of lese majeste would be tried in military courts, and that they could not be appealed. The military-installed administration declared defense of the monarchy a priority, and in addition to vigorously pursuing prosecutions at home, it has vowed to seek the return of critics abroad it considers to have insulted the monarchy.

On March 20, a military court sentenced a 67-year-old man to 18 months in prison on charges of writing insults about the king in a shopping mall's bathrooms, and in February, two theater activists were sentenced to two and a half years in prison on charges of insulting the monarchy in a play they produced about a fictional king and his adviser.